1. Field of the Invention
The present invention relates to a compressing device and method, a decompressing device and method, a compressing/decompressing system, a program, and a record medium and particularly concerns methods of compressing and decompressing successive analog signals or discrete digital signals.
2. Description of the Related Art
Conventionally, in case of transmitting and accumulating a signal such as an image signal and an aural signal having a large amount of information, signals have been compressed and decompressed in order to reduce an amount of transmitted information and increase storable time in a storage medium. In general, when an analog signal is compressed, the analog signal is firstly sampled according to a predetermined sampling frequency and is digitalized, and a compressing operation is performed on obtained digital data.
Thinning compression has been present as an example of the compressing operation. The most simple method of thinning compression is that data on a predetermined sample point is discarded from all the sample points (conversely, only data on the predetermined sample point is used). For example, a double compression ratio can be achieved by adopting data on alternate sample points. When a compression ratio can be increased, data is used on every nth (>2) sample points.
According to this method, although signals can be compressed without performing any operations at all, decompression causes a problem of reproducibility to the original data. Namely, on the decompression part, data between sample points adopted during compression is obtained by an interpolating operation. More data are discarded on sample points that are originally required for accurately reproducing the original data as adopted sample points have a larger interval to increase a compression ratio. Thus, reproducibility to the original data is considerably degraded.
For this reason, in order to attain both of an improved compression ratio and reproducibility to the original data, an amount of information is normally thinned out by performing some operation on data on sample points. For example, a method of replacing data A and B on two adjacent sample points with an average data (A+B)/2 is available. With this method, unlike the case where one of the original data A and B is completely abandoned, since data added with both of the original data A and B is adopted as compression data (thinned-out data), reproducibility to the original data is somewhat improved.
However, such an operation cannot obtain any preferable reproducibility sufficiently for practical use. In order to increase a compression ratio and obtain data reproducibility sufficiently for practical use, more complicated operations are necessary.
Further, when thinning compression is performed, return noise is caused by degraded frequency characteristics and degrades the quality of signals obtained by decompression.
As described above, it has been extremely difficult to improve both of a compression ratio and the quality of reproduced data. In such a state, methods of compression and decompression for performing complicated operations are currently used instead of simple thinning compression in many cases.
For example, for compression of an image signal and an aural signal, the following method is used: after the original data is processed by using a conversion filter of a time axis—frequency axis of a DCT (Discreat-Cosine-Transform) and so on, compression is performed in a frequency area. DPCM (Differential Pulse Code Modulation) frequently used as a compressing method of an aural signal on a telephone line has been used in consideration of this point. Besides, the compressing method of DPCM is a method of coding a difference between adjacent sample values when a waveform is sampled.
As a method of time/frequency conversion, a method using a sub-band filter and MDCT (Modified Discrete Cosine Transform) is available, and MPEG (Moving Picture Image Coding Experts Group) audio is available as a coding method using such a method.
Further, an image compressing system used most widely has been generally known as an MPEG standard.
A decompressing operation for data compressed according to the above compressing method is basically performed according to the reversed operation of the compressing operation of the same compressing method.
Namely, compressed digital data is converted from a signal of a frequency area to a signal of a time area by frequency/time conversion processing, and then, a predetermined decompressing operation is performed, so that the original digital data is reproduced. Then, the original data obtained thus is subjected to digital-analog conversion at need, and the data is outputted as an analog signal.
In recent years, an amount of information has increased as an image signal and an aural signal become finer, and information communication using a mobile terminal, an Internet, and so on has become widespread. Thus, a higher compression ratio and the higher quality of reproduced data have been demanded increasingly. However, although improvement has been made in the above described conventional compressing and decompressing methods using DCT or the like, there is a limit on a higher compression ratio and reproduced data with higher quality. Hence, it has been extremely difficult to realize a higher compression ratio and higher quality.
Further, in the above described conventional compressing and decompressing methods, since a signal on a time base is converted to a signal on a frequency axis before compression, processing such as time/frequency conversion during compression and frequency/time conversion during decompression is necessary. Thus, there is a problem that the processing is complicated and the configuration for the processing is extremely complicated, resulting in difficulty in achieving a smaller device as well as longer processing time of compression and decompression.
The present invention is achieved to solve the above-described problems and has as its objective the provision of completely new compressing and decompressing methods of improving a compressing ratio and the quality of reproduced data.
Moreover, the present invention is also aimed to simplify the compressing and decompressing operations of a signal to shorten processing time and also simplify the configuration for performing the operations.